The human spine is a flexible weight bearing column formed from a plurality of bones called vertebrae. There are 33 vertebrae which are named based on which of five regions (cervical, dorsal, lumbar, sacral, and coccygeal) in which they are found. Going from the top of the spine down, in general there are seven cervical vertebra, twelve dorsal vertebra, five lumbar vertebra, five sacral vertebra, and four coccygeal vertebra. The vertebra of the cervical, dorsal, and lumbar regions of the spine are separate throughout the life of an individual, but the vertebra of the sacral and coccygeal regions in an adult are fused to form two bones, the five sacral vertebra entering into the formation of the sacrum and the four coccygeal vertebra into the coccyx. In general, each vertebra contains an anterior, solid segment or body and a posterior segment or arch. The arch is generally formed of two pedicles and two laminae, supporting seven processes--four articular, two transverse, and one spinous. There are exceptions to these general characteristics of a vertebra. For example, the first cervical vertebra (atlas vertebra) has neither a body nor spinous process. Also, the second cervical vertebra (axis vertebra) has an odontoid process, which is a strong, prominent process, shaped like a tooth, rising perpendicularly from the upper surface of the body of the axis vertebra. Further details regarding the construction of the spine are known to those of ordinary skill in the art and may be found in such common references as Gray's Anatomy, Crown Publishers, Inc., 1977, pp. 33-54, which is herein incorporated by reference.
The past two decades have seen greatly increased use of implants for the stabilization of fractures and/or fusion of various portions of the spine. These implant devices include a variety of longitudinal elements such as rods or plates which span two or more vertebra and are affixed to the vertebra by various fixation elements such as wires, staples, and screws (often inserted through the pedicles of the vertebra). These systems may be affixed to either the posterior or the anterior side of the spine. In many cases, these implant systems are prominent beneath the skin and have a higher profile than more simple fixation devices. One such simpler fixation device is in the stable posterior fusion of the atlas and axis vertebra by transarticular screw fixation using the technique of Magerl et al. disclosed in Stable Posterior Fusion of the Atlas and Axis by Transarticular Screw Fixation, F. Magerl, P-S. Seeman, Cervical Spine, Volume 1, Springer-Verlag, Copyright 1987, pp. 322-327; Primary Posterior Fusion 1-2 in Odontoid Factors; Indications, Technique, and Results of Transarticular Screw Fixation, B. Jeanneret and F. Magerl, Journal of Spinal Disorders, Volume 5, No. 4, pp. 464-475, 1992, Raven Press, Ltd., New York; (also see Atlanto-Axial Fusion With Transarticular Screw Fixation, D. Grob, B. Jeanneret, M. Aebi, and T. M. Markwalder, The Journal of Bone and Joint Surgery Volume 73-B, No. 6, 1991, pp. 972-976 all of which are herein incorporated by reference.
The use of transarticular screw fixation in both fusion procedures and stabilization procedures for fractures has undergone increasing use. However, due to the small entry angle of the screw with respect to the back of a patient lying prone on the operating table, procedures making use of transarticular screw fixation have required extremely long and wide midline incisions in order to place the screws as necessary in various procedures in both the cervical and lumbar spine regions. These large incisions result in increased operating time with consequent increase in blood loss as well as enlarging the size of the scar left on the patient. It should be understood that while reduction of pain and maintaining range of motion are the surgical goal, the size of the incision and the scar it leaves behind are often the only visible measure a patient will have to judge the quality of the surgeon's work. Thus, it is preferable if the incision is made in a manner not only to preserve the skin's contour, but of a minimum length and size to increase patient satisfaction.